In a cellular radio communication network such as a mobile phone network, a wide service area is formed by combining many small zones called cells. Those cells are generally managed by one radio base station in units of one to six cells. Further, the adjacent cells generally have an overlap in their coverage areas.
An operator of the radio communication network performs cell coverage design for providing high quality radio communication services to subscribers. In general, a drive test that makes measurements within an area with a vehicle equipped with a dedicated measurement device is conducted to make a survey on the place where the radio quality is not sufficient (coverage hole), the place where there is strong interference from many cells (pilot pollution) and the like. Then, the operator adjusts a radio parameter which can change cell coverage of a radio base station so as to solve the problems found in the survey. Examples of radio parameter which can change the cell coverage include transmission power of a pilot signal or a reference signal from a base station, and tilt angle of a base station antenna. Cell coverage varies depending on changes in the surrounding environment, such as installation of a new base station and construction of a building, for example. Therefore, the operator needs to perform coverage design on a regular basis even after a radio base station is installed. To reduce a cost of such coverage design, techniques to autonomously optimize cell coverage are proposed (for example, Patent Literatures 1 and 2).
According to the technique disclosed in Patent Literature 1, a control apparatus, placed in a network including a cell, collects measurement reports containing measurement results of radio quality of the cell (specifically, received power of a pilot signal that defines the cell coverage) from mobile stations and calculates proportion of the measurement reports indicating that the radio quality is equal to or less than a threshold. Then, the control apparatus adjusts the transmission power of the pilot signal so that the proportion becomes a specified value. For example, when the proportion of measurement reports where the radio quality is equal to or less than the threshold is relatively low as a result of summarizing measurement reports from mobile stations, the control apparatus determines that there is no sufficient overlap with neighboring cells and increases the transmission power of the pilot signal. On the contrary, when the proportion of measurement reports where the radio quality is equal to or less than the threshold is high, the control apparatus determines that there is excessive overlap with neighboring cells (i.e., interference is occurring) and decreases the transmission power of the pilot signal.
According to the technique disclosed in Patent Literature 2, a base station collects, from mobile stations, reports indicating the number of neighboring cells detected at cell edge of its cell, then the base station adjusts downlink transmission power of its cell so that the number of neighboring cells detected at the cell edge becomes three. Specifically, when the number of neighboring cells detected at the cell edge is two or less, the base station determines that there is no sufficient overlap between its cell and neighboring cells and increases the downlink transmission power. On the contrary, when the number of neighboring cells detected at the cell edge is four or more, the base station determines that there is excessive overlap between its cell and neighboring cells and decreases the downlink transmission power.